Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development. They are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and stimulation with growth factors such as transforming growth factors, interleukins and platelet-derived growth factor.
In mammals, VEGFs are encoded by a family of genes and include VEGF-A, VEGF-B, VEGF-C, VEGF-D and Placenta like Growth Factor (PlGF). Highly related proteins include orf virus-encoded VEGF-like proteins referred to as VEGF-E and a series of snake venoms referred to as VEGF-F. VEGFs and VEGF-related proteins are members of the Platelet Derived Growth Factor (PDGF) supergene family of cystine knot growth factors. All members of the PDGF supergene family share a high degree of structural homology with PDGF (see U.S. patent application Ser. No. 09/813,398 which is herein incorporated by reference in its entirety).
VEGF-A, VEGF-B and PlGF are predominantly required for blood vessel formation, whereas VEGF-C and VEGF-D are essential for the formation of lymphatic vessels. Angiogenesis is the process by which new blood vessels or lymphatic vessels form by developing from pre-existing vessels. The process is initiated when VEGFs bind to receptors on endothelial cells, signaling activation of endothelial cells. Activated endothelial cells produce enzymes which dissolve tiny holes in the basement membrane surrounding existing vessels. Endothelial cells then begin to proliferate and migrate out through the dissolved holes of the existing vessel to form new vascular tubes (Alberts et al., 1994, Molecular Biology of the Cell. Garland Publishing, Inc., New York, N.Y. 1294 pp.).
Three type III receptor tyrosine kinases are activated by VEGFs during angiogenesis: fms-like tyrosine kinase (Flt-1, also known as VEGFR1), kinase domain receptor or kinase insert domain-containing receptor (KDR, also known as VEGFR2 and Flk-1) and Flt-4 (also known as VEGFR3). KDR is the predominant receptor in angiogenic signaling, whereas Flt-1 is associated with the regulation of blood vessel morphogenesis and Flt-4 regulates lymphangiogenesis. These receptors are expressed almost exclusively on endothelial cells, with a few exceptions such as the expression of Flt-1 in monocytes where it mediates chemotaxis (Barleon et al., 1996, Blood. 87: 3336-3343).
VEGF receptors are closely related to Fms, Kit and PDGF receptors. They consist of seven extracellular immunoglobulin (Ig)-like domains, a transmembrane (TM) domain, a regulatory juxtamembrane domain, an intracellular tyrosine kinase domain interrupted by a short peptide, the kinase insert domain, followed by a sequence carrying several tyrosine residues involved in recruiting downstream signaling molecules. Mutation analysis of the extracellular domains of Flt-1 and KDR show that the second and third Ig-like domains constitute the high-affinity ligand-binding domain for VEGF with the first and fourth Ig domains apparently regulating ligand binding and receptor dimerization, respectively (Davis-Smyth et al., 1998, J. Biol. Chem. 273: 3216-3222; Fuh et al., 1998, J. Biol. Chem. 273: 11197-11204; and Shinkai et al., 1998, J. Biol. Chem. 273: 31283-31288). Receptor tyrosine kinases are activated upon ligand-mediated receptor dimerization (Hubbard, 1991, Prog. Biophys. Mol. Biol. 71: 343-358; Jiang and Hunter, 1999, Curr. Biol. 9: R568-R571; and Lemmon and Schlessinger, 1998, Methods Mol. Biol. 84: 49-71). Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparin sulfate, integrins or cadherins.
VEGF molecules interact with one or more tyrosine kinase receptors during angiogenesis. For instance, VEGF-A acts predominantly through KDR and Flt-1. VEGF-C and VEGF-D similarly are specific ligands for KDR and VEGFR3. PlGF and VEGF-B are believed to bind only to Flt-1. Viral VEGF-E variants activate KDR. VEGF-F variants interact with either VEGFR3 or KDR.
In addition to the two classical receptors, there are several membrane or soluble receptors modulating VEGF bioactivity and angiogenesis. For instance, neuropilin-1 and neuropilin-2 interact with both KDR and Flt-1, respectively, stimulating signaling of those receptors. Isoforms of VEGF-A, VEGF-B, PlGF-2 have been shown to bind to neuropilin-1 (Soker et al., 1998, Cell. 92: 735-745; Makinen et al., 1999, J. Biol. Chem. 274: 21217-21222; and Migdal et al., 1998, J. Biol. Chem. 273: 22272-22278). VEGF isoforms capable of interacting of interacting with neuropilin, i.e., those isoforms with exon 7 or 6 and 7, are also capable of interacting with heparin sulfate.
Although VEGF-A is the best characterized of the VEGF proteins, the molecular basis of the interaction between VEGF-A and KDR and Flt-1 is not well understood. Although VEGFR1 binds VEGF-A with a 50-fold higher affinity than KDR, KDR is considered to be the major transducer of VEGF-A angiogenic effects, i.e., mitogenicity, chemotaxis and induction of tube formation (Binetruy-Tourniere et al., supra). There is, however, growing evidence that Flt-1 has a significant role in hematopoiesis and in the recruitment of monocytes and other bone-marrow derived cells that may home in on tumor vasculature and promote angiogenesis (Hattori et al., 2002, Nature Med. 8: 841-849; Gerber et al., 2002, Nature. 417: 954-958; and Luttun et al., 2002, Nature Med. 8: 831-840). Further, in some cases Flt-1 is expressed by tumor cells and may mediate a chemotactic signal, thus potentially extending the role of this receptor in cancer growth (Wey et al., 2005, Cancer. 104: 427-438).
A single VEGF-A homodimer induces dimerization of two KDR receptors and autophosphorylation of their cytoplasmatic portions. Previous studies suggested that by analogy to glycoprotein hormones, the charged amino acid residues in the peripheral loops of VEGF-A are also critical in providing high affinity electrostatic interactions with its respective receptors (Szkudlinski et al., 1996, Nat. Biotechnol. 14(10): 1257-63; Fuh et al., supra; Muller et al., 1997, Proc. Natl. Acad. Sci. U.S.A. 94(14): 7192-7; Keyt et al., 1996, J. Biol. Chem. 271(10): 5638-46). However, it should be noted that many mutations in VEGF-A have no major effect on receptor binding affinity. Mutations in the peripheral loops of VEGF primarily have resulted in loss-of-function. Further, there appear to be no previous amino acid substitutions increasing binding affinity to KDR more than 2-fold.
Angiogenesis is responsible for beneficial biological events such as wound healing, myocardial infarction repair, and ovulation. On the other hand, angiogenesis is also responsible for causing or contributing to diseases such as growth and metastasis of solid tumors (Isayeva et al., 2004, Int. J. Oncol. 25(2):335-43; Takeda et al., 2002, Ann Surg. Oncol. 9(7):610-16); atherosclerosis; abnormal neovascularization of the eye as seen in diseases such as retinopathy of prematurity, diabetic retinopathy, retinal vein occlusion, and age-related macular degeneration (Yoshida et al., 1999, Histol Histopathol. 14(4):1287-94; Aiello, 1997, Ophthalmic Res. 29(5):354-62); chronic inflammatory conditions such as rheumatoid arthritis osteoarthritis, and septic arthritis; neurodegenerative disease (Ferrara, N., 2004, Endocr. Rev. 25: 581-611); placental insufficiency, i.e., preeclampsia (Ferrara, supra); and skin diseases such as dermatitis, psoriasis, warts, cutaneous malignancy, decubitus ulcers, stasis ulcers, pyogenic granulomas, hemangiomas, Kaposi's sarcoma, hypertrophic scars, and keloids (Arbiser, 1996, J. Am. Acad. Dermatol. 34(3):486-97). During rheumatoid arthritis, for example, endothelial cells become activated and express adhesion molecules and chemokines, leading to leukocyte migration from the blood into the tissue. Endothelial cell permeability increases, leading to edema formation and swelling of the joints (Middleton et al., 2004, Arthritis Res. Ther. 6(2):60-72).
VEGF, in particular VEGF-A, has been implicated in many of the diseases and conditions associated with increased, decreased, and/or dysregulated angiogenesis (Binetruy-Tourniere et al., 2000, EMBO J. 19(7): 1525-33). For instance, VEGF has been implicated in promoting solid tumor growth and metastasis by stimulating tumor-associated angiogenesis (Lu et al., 2003, J. Biol. Chem. 278(44): 43496-43507). VEGF is also a significant mediator of intraocular neovascularization and permeability. Overexpression of VEGF in transgenic mice results in clinical intraretinal and subretinal neovascularization, and the formation of leaky intraocular blood vessels detectable by angiography, as seen in human eye disease (Miller, 1997, Am. J. Pathol. 151(1): 13-23). Additionally, VEGF has been identified in the peritoneal fluid of women with unexplained infertility and endometriosis (Miedzybrodzki et al., 2001, Ginekol. Pol. 72(5): 427-430), and the overexpression of VEGF in testis and epididymis has been found to cause infertility in transgenic mice (Korpelainen et al., 1998, J. Cell Biol. 143(6): 1705-1712). Recently, VEGF-A has been identified in the synovial fluid and serum of patients with rheumatoid arthritis (RA), and its expression is correlated with disease severity (Clavel et al., 2003, Joint Bone Spine. 70(5): 321-6). Given the involvement of pathogenic angiogenesis in such a wide variety of disorders and diseases, inhibition of angiogenesis, and particularly of VEGF signaling, is a desirable therapeutic goal.
Inhibition of angiogenesis and tumor inhibition has been achieved by using agents that either interrupt VEGF-A and KDR interaction and/or block the KDR signal transduction pathway including: peptides that block binding of VEGF to KDR (Binetruy-Tourniere et al., 2000, EMBO J. 19(7): 1525-33); antibodies to VEGF (Kim et al., 1993, Nature 362, 841-844; Lanai et al., 1998, J. Cancer 77, 933-936; Margolin et al., 2001, J. Clin. Oncol. 19, 851-856); antibodies to KDR (Lu et al., 2003, supra; Zhu et al., 1998, Cancer Res. 58, 3209-3214; Zhu et al. 2003, Leukemia 17, 604-611; Prewett et al., 1999, Cancer Res. 59, 5209-5218); soluble receptors (Holash et al., 2002, Proc. Natl. Acad. Sci. USA 99, 11393-11398; Clavel et al. supra); tyrosine kinase inhibitors (Fong et al., 1999, Cancer Res. 59, 99-106; Wood et al., 2000, Cancer Res. 60, 2178-2189; Grosios et al., 2004, Inflamm Res. 53(4):133-42); anti-VEGF immunotoxins (Olson et al., 1997, Int. J. Cancer 73, 865-870); ribozymes (Pavco et al., 2000, Clin. Cancer Res. 6, 2094-2103); antisense mediated VEGF suppression (Forster et al., 2004, Cancer Lett. 20; 212(1):95-103); RNA interference (Takei et al., 2004, Cancer Res. 64(10):3365-70; Reich et al., 2003, Mol. Vis. 9:210-6); and undersulfated, low molecular weight glycol-split heparin (Pisano et al., 2005, Glycobiology. 15(2) 1-6). Some of these treatments, however, have resulted in undesirable side effects. For instance, Genentech's Avastin, a monoclonal antibody that targets VEGF, has been reported to cause an increase in serious arterial thromboembolic events in some colon cancer patients and serious, and in some cases even fatal, hemoptysis in non-small cell lung cancer patients (Ratner, 2004, Nature Biotechnol. 22(10):1198). More recently, Genentech has reported that gastrointestinal perforations were observed in 11% of ovarian cancer patients (5 women out of 44 in trial) treated with Avastin (Genentech Press Release dated Sep. 23, 2005). Similarly, the first VEGF-targeting drug, Pfizer's receptor tyrosine kinase inhibitor SU5416, exhibited severe toxicities that included thromboembolic events, prompting Pfizer to discontinue development (Ratner, supra). Given the wide variety of patients that stand to benefit from the development of effective anti-angiogenic treatments and the drawbacks of some known anti-angiogenesis treatments, there remains a need for novel anti-angiogenic therapeutics.